In parameter control systems, such as those used for controlling pressure in an industrial process, the control system monitors the magnitude of an input parameter (e.g., pressure) and tries to maintain the input parameter within an acceptable range of a desired magnitude of that parameter. Since the controlled parameter is affected by other environmental factors (e.g., temperature, material inputs to the processor, etc.), it is necessary for the control system to respond to the changes caused by these factors in order to maintain the parameter within the desired range of magnitude. Furthermore, the desired range of magnitude of the parameter can change during a process to a new desired range. If this occurs, the control system must be able to bring the input parameter to a magnitude within the desired range and hold it at that magnitude. A control system must be able to perform its control functions with a fast response time, with a minimum of overshoot (or undershoot), without hunting to achieve the new desired magnitude, and without surging in response to changes in the system.
In typical prior art systems, a number of factors determine how well the systems maintain the controlled parameter at a desired magnitude and how well the systems respond to changes in the process which they are controlling. Most systems have adjustments for the gain of the system which determines the magnitude of the response of the system to error signals (i.e., the difference between the desired magnitude of the controlled parameter and the measured magnitude of the controlled parameter). A typical control system also has adjustments for the amount of phase lag between changes in the parameter being controlled and the output signal to a device, such as a valve, which controls the parameter. The phase lag provides an integrating factor which causes the system to be more responsive to changes in the measured parameter. Many control systems also include an adjustment for phase lead or differentiation which allows the control system to anticipate when the magnitude of the controlled parameter is going to be within an acceptable range of the desired magnitude, and thus act to change the control signals to the controlling device so that overshoot (or undershoot) of the desired magnitude is avoided.
Many prior art systems having the capabilities discussed above can be adjusted to provide accurate control of the controlled parameter. However, the adjustments which may be optimal for one range of magnitudes of the controlled parameter are typically not optimal for other ranges. Thus, if the desired magnitude of the controlled parameter changes during the process (e.g., if the first segment of a process is performed at one pressure level and second and subsequent segments are performed at different pressure levels), then the operator must make new adjustments to maintain optimal performance of the control system.
The adjustments described above are typically dependent upon system characteristics which can change with time. For example, in a pressure control system, a pump which causes the flow of a gas or other fluid into or out of the process chamber can vary in performance over time. Although the adjustments in the gain, phase lag and phase lead may provide optimal performance initially, changes in the characteristics of the system require that the adjustments be varied repeatedly to maintain optimal performance of the control system. Furthermore, a means must be provided to determine what the new characteristics are in order to determine what the adjustments to the gain, phase lag and phase lead should be. This can be accomplished by testing the system being controlled and measuring its response at various magnitudes of the controlled parameter and then calculating the adjustments required to optimize the performance. This is sometimes done prior to operating the system so that the information pertaining to the response characteristics at various magnitudes is available for making the adjustments required during the process.
Some control systems are available which are responsive to changes in the characteristics of the controlled system which may occur after a process is started. These control systems typically use the technique of periodically introducing a disturbance to the controlled system so that the ability of the control system to effect changes in the controlled parameter can be measured and the results of the measurements used to optimize the settings of the gain, phase lead and phase lag. However, inherent in this technique is the disturbance of an operating system. Rather than continuously driving the controlled parameter towards the desired magnitude of the parameter, the disturbance systems periodically drive the controlled parameter away from the desired magnitude. Many processes are sensitive to the controlled parameter and cannot tolerate the temporary changes introduced by the disturbance techniques.
Control systems such as those described above are typically adjusted for a limited range of process system configurations. If the control system is used on a different process system, the control system must be readjusted for the physical parameters of the different process system. For example, the characteristics of a process system having a vacuum chamber of one volume will have substantially different characteristics than a process system with a significantly larger or significantly smaller vacuum chamber. A typical prior art control system is not readily useable for controlling a different process system without requiring adjustments to its gain, phase lag and phase lead characteristics.
A need thus exists for a control system which does not require operator input to adjust the control system for optimal control of a parameter. The control system should automatically compensate for changes in the desired magnitude of the controlled parameter and should also compensate for changes in the characteristics of the process which is being controlled, including changes in the physical parameters of the system in which the process is occurring. The control system should accomplish the foregoing without introducing any disturbances to the process which is being controlled.